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Jun 30, 2018
06/18

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Ralf Lehnert

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Violations of Lorentz symmetry are typically associated with modifications of one-particle dispersion relations. The physical effects of such modifications in particle collisions often grow with energy, so that ultrahigh-energy cosmic rays provide an excellent laboratory for measuring such effects. In this talk we argue that collisions at particle colliders, which involve much smaller energies, can nevertheless yield competitive constraints on Lorentz breaking.

Topics: High Energy Physics - Phenomenology, High Energy Physics - Experiment

Source: http://arxiv.org/abs/1403.6051

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Sep 22, 2013
09/13

by
Ralf Lehnert

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In this talk, we explore the relation between smoothly varying couplings and Lorentz violation. Within the context of a supergravity model, we present an explicit mechanism that causes the effective fine-structure parameter and the effective electromagnetic $\theta$ angle to acquire related spacetime dependences. We argue that this leads to potentially observable Lorentz violation and discuss some implications for the Standard-Model Extension.

Source: http://arxiv.org/abs/hep-ph/0308208v1

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Jun 30, 2018
06/18

by
Ralf Lehnert

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We prove the existence of Veech groups having a critical exponent strictly greater than any elementary Fuchsian group (i.e. $>\frac{1}{2}$) but strictly smaller than any lattice (i.e. $

Topics: Dynamical Systems, Mathematics, Geometric Topology

Source: http://arxiv.org/abs/1404.2110

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Jun 29, 2018
06/18

by
Ralf Lehnert

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Various approaches to physics beyond the Standard Model can lead to small violations of CPT invariance. Since CPT symmetry can be measured with ultrahigh precision, CPT tests offer an interesting phenomenological avenue to search for underlying physics. We discuss this reasoning in more detail, comment on the connection between CPT and Lorentz invariance, and review how CPT breaking would affect the (anti)hydrogen spectrum.

Topic: High Energy Physics - Phenomenology

Source: http://arxiv.org/abs/1604.08419

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Sep 21, 2013
09/13

by
Ralf Lehnert

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Remnant low-energy effects of Planck-scale Lorentz breaking in candidate fundamental theories typically include modified one-particle dispersion relations. Theoretical constraints on such modifications are discussed leading to the exclusion of a variety of previously considered Lorentz-violating parameters. In particular, the fundamental principle of coordinate independence, the role of an effective dynamical framework, and the conditions of positivity and causality are investigated.

Source: http://arxiv.org/abs/hep-ph/0402133v1

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Sep 21, 2013
09/13

by
Ralf Lehnert

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Cosmological models often contain scalar fields, which can acquire global nonzero expectation values that change with the comoving time. Among the possible consequences of these scalar-field backgrounds, an accelerated cosmological expansion, varying couplings, and spacetime-symmetry violations have recently received considerable attention. This talk studies the interplay of these three key signatures of cosmologically varying scalars within a supergravity framework.

Source: http://arxiv.org/abs/hep-ph/0508316v1

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Sep 19, 2013
09/13

by
Ralf Lehnert

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Small violations of spacetime symmetries have recently been identified as promising Planck-scale signals. This talk reviews how such violations can arise in various approaches to quantum gravity, how the emergent low-energy effects can be described within the framework of relativistic effective field theories, how suitable tests can be identified, and what sensitivities can be expected in current and near-future experiments.

Source: http://arxiv.org/abs/gr-qc/0701006v1

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Sep 18, 2013
09/13

by
Ralf Lehnert

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The breakdown of spacetime symmetries has recently been identified as a promising candidate signal for underlying physics, possibly arising through quantum-gravitational effects. This talk gives an overview over various aspects of CPT- and Lorentz-violation research. Particular emphasis is given to the interplay between CPT, Lorentz, and translation symmetry, mechanisms for CPT and Lorentz breaking, and the construction of a low-energy quantum-field description of such effect. This quantum...

Source: http://arxiv.org/abs/hep-ph/0611177v2

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Jun 30, 2018
06/18

by
Ralf Lehnert

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An important open question in fundamental physics concerns the nature of spacetime at distance scales associated with the Planck length. The widespread belief that probing such distances necessitates Planck-energy particles has impeded phenomenological and experimental research in this context. However, it has been realized that various theoretical approaches to underlying physics can accommodate Planck-scale violations of spacetime symmetries. This talk surveys the motivations for...

Topic: High Energy Physics - Phenomenology

Source: http://arxiv.org/abs/1404.3670

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Jun 29, 2018
06/18

by
Ralf Lehnert

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Lorentz and CPT invariance are among the symmetries that can be investigated with ultrahigh precision in subatomic physics. Being spacetime symmetries, Lorentz and CPT invariance can be violated by minuscule amounts in many theoretical approaches to underlying physics that involve novel spacetime concepts, such as quantized versions of gravity. Regardless of the underlying mechanism, the low-energy effects of such violations are expected to be governed by effective field theory. This talk...

Topic: High Energy Physics - Phenomenology

Source: http://arxiv.org/abs/1604.05898

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Sep 21, 2013
09/13

by
Ralf Lehnert

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Many theoretical approaches to quantum gravity predict the breakdown of Lorentz symmetry at Planck energies. Kinematical cosmic-ray studies are a sensitive tool in the search for such effects. This talk discusses the construction of test dispersion relations for such analyses.

Source: http://arxiv.org/abs/astro-ph/0508625v1

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Sep 18, 2013
09/13

by
Ralf Lehnert

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The modified Dirac equation in the Lorentz-violating Standard-Model Extension (SME) is considered. Within this framework, the construction of a hermitian Hamiltonian to all orders in the Lorentz-breaking parameters is investigated, discrete symmetries and the first-order roots of the dispersion relation are determined, and various properties of the eigenspinors are discussed.

Source: http://arxiv.org/abs/hep-ph/0401084v1

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Sep 17, 2013
09/13

by
Ralf Lehnert

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The Chern-Simons-type term in the photon sector of the Lorentz- and CPT-breaking minimal Standard-Model Extension (mSME) is considered. It is argued that under certain circumstances this term can be removed from the mSME. In particular, it is demonstrated that for lightlike Lorentz violation a field redefinition exists that maps the on-shell free Chern-Simons model to conventional on-shell free electrodynamics. A compact explicit expression for an operator implementing such a mapping is...

Source: http://arxiv.org/abs/0711.4851v2

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Sep 23, 2013
09/13

by
Ralf Lehnert

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The violation of Lorentz symmetry can arise in a variety of approaches to fundamental physics. For the description of the associated low-energy effects, a dynamical framework known as the Standard-Model Extension has been developed. This talk gives a brief review of the topic focusing on Lorentz violation through varying couplings.

Source: http://arxiv.org/abs/hep-ph/0312093v1

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Sep 22, 2013
09/13

by
Ralf Lehnert

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We investigate the compatibility of Lorentz-violating quantum field theories with the requirements of causality and stability. A general renormalizable model for free massive fermions indicates that these requirements are satisfied at low energies provided the couplings controlling the breaking are small. However, for high energies either microcausality or energy positivity or both are violated in some observer frame. We find evidence that this difficulty can be avoided if the model is...

Source: http://arxiv.org/abs/hep-th/0201238v1

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Sep 21, 2013
09/13

by
Ralf Lehnert

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In the context of N=4 supergravity in four dimensions, we present an exact classical solution that leads to spacetime-dependent electromagnetic couplings and discuss the ensuing Lorentz-violating effects. We comment briefly on experimental bounds.

Source: http://arxiv.org/abs/hep-ph/0311073v1

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Sep 18, 2013
09/13

by
Ralf Lehnert

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Some motivations for Lorentz-symmetry tests in the context of quantum-gravity phenomenology are reiterated. The description of the emergent low-energy effects with the Standard-Model Extension (SME) is reviewed. The possibility of constraining such effects with dispersion-relation analyses of collider data is established.

Source: http://arxiv.org/abs/1102.2923v1

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Sep 18, 2013
09/13

by
Ralf Lehnert

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Quantum-gravity effects are expected to be suppressed by the Planck mass. For experimental progress it is therefore important to identify potential signatures from Planck-scale physics that are amenable to ultrahigh-precision tests. It is argued that minuscule violations of Lorentz and CPT symmetry are candidate signals. In addition, theoretical and experimental aspects of the Standard-Model Extension, which describes the emergent low-energy effects, are discussed.

Source: http://arxiv.org/abs/hep-ph/0401124v1

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Sep 21, 2013
09/13

by
Ralf Lehnert

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Lorentz and CPT tests involving matter-antimatter comparisons at low temperatures are discussed. SME predictions for transition frequencies in such systems include both matter-antimatter differences and sidereal variations. In hydrogen-antihydrogen spectroscopy, leading-order effects in a 1S-2S transition as well as in a 1S Zeeman transition could exist that can be employed to obtain clean constraints. Similarly, tight bounds can be determined from Penning-trap experiments involving antiprotons.

Source: http://arxiv.org/abs/hep-ph/0602097v1

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Sep 23, 2013
09/13

by
Ralf Lehnert

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The Lorentz- and CPT-violating Chern-Simons extension of electrodynamics is considered. In the context of N=4 supergravity in four spacetime dimensions, it is argued that cosmological solutions can generate this extension. Within Chern-Simons electrodynamics, theoretical and phenomenological topics are reviewed that concern the number of the remaining spacetime symmetries and the vacuum Cherenkov effect, respectively.

Source: http://arxiv.org/abs/0710.0411v1

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Sep 21, 2013
09/13

by
Ralf Lehnert

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This talk discusses the relation between spacetime-dependent scalars, such as couplings or fields, and the violation of Lorentz symmetry. A specific cosmological supergravity model demonstrates how scalar fields can acquire time-dependent expectation values. Within this cosmological background, excitations of these scalars are governed by a Lorentz-breaking dispersion relation. The model also contains couplings of the scalars to the electrodynamics sector leading to the time dependence of both...

Source: http://arxiv.org/abs/gr-qc/0508122v1

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Jul 20, 2013
07/13

by
Ralf Lehnert

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This work instigates a study of non-local field mappings within the Lorentz- and CPT-violating Standard-Model Extension (SME). An example of such a mapping is constructed explicitly, and the conditions for the existence of its inverse are investigated. It is demonstrated that the associated field redefinition can remove b-type Lorentz violation from free SME fermions in certain situations. These results are employed to obtain explicit expressions for the corresponding Lorentz-breaking...

Source: http://arxiv.org/abs/hep-th/0609162v1

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Sep 19, 2013
09/13

by
Ralf Lehnert

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In the context of threshold investigations of Lorentz violation, we discuss the fundamental principle of coordinate invariance, the role of an effective dynamical framework, and the conditions of positivity and causality. Our analysis excludes a variety of previously considered Lorentz-breaking parameters and opens an avenue for viable dispersion-relation investigations of Lorentz violation.

Source: http://arxiv.org/abs/gr-qc/0304013v1

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Sep 21, 2013
09/13

by
Ralf Lehnert

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A number of approaches to fundamental physics can lead to the violation of Lorentz and CPT symmetry. This talk discusses the low-energy phenomenology associated with such effects and reviews various sample experiments within this context.

Source: http://arxiv.org/abs/1008.1746v1

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Sep 21, 2013
09/13

by
Ralf Lehnert

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The violation of spacetime symmetries provides a promising candidate signal for underlying physics, possibly arising at the Planck scale. This talk gives an overview over various aspects in the field, including some mechanisms for Lorentz breakdown, the SME test framework, and phenomenological signatures for such effects.

Source: http://arxiv.org/abs/gr-qc/0602073v1

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Sep 19, 2013
09/13

by
Ralf Lehnert

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Violations of spacetime symmetries have recently been identified as promising signatures for physics underlying the Standard Model. The present talk gives an overview over various topics in this field: The motivations for spacetime-symmetry research, including some mechanisms for Lorentz breaking, are reviewed. An effective field theory called the Standard-Model Extension (SME) for the description of the resulting low-energy effects is introduced, and some experimental tests of Lorentz and CPT...

Source: http://arxiv.org/abs/gr-qc/0703089v1

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Sep 18, 2013
09/13

by
Ralf Lehnert

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One of the most difficult questions in present-day physics concerns a fundamental theory of space, time, and matter that incorporates a consistent quantum description of gravity. There are various theoretical approaches to such a quantum-gravity theory. Nevertheless, experimental progress is hampered in this research field because many models predict deviations from established physics that are suppressed by some power of the Planck scale, which currently appears to be immeasurably small....

Source: http://arxiv.org/abs/1102.3612v1

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Jul 20, 2013
07/13

by
Ralf Lehnert; Robertus Potting

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Within the classical Maxwell-Chern-Simons limit of the Standard-Model Extension (SME), the emission of light by uniformly moving charges is studied confirming the possibility of a Cherenkov-type effect. In this context, the exact radiation rate for charged magnetic point dipoles is determined and found in agreement with a phase-space estimate under certain assumptions.

Source: http://arxiv.org/abs/hep-ph/0406128v1

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Jun 27, 2018
06/18

by
Joshua Foster; Ralf Lehnert

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The classical propagation of certain Lorentz-violating fermions is known to be governed by geodesics of a four-dimensional pseudo-Finsler $b$ space parametrized by a prescribed background covector field. This work identifies systems in classical physics that are governed by the three-dimensional version of Finsler $b$ space and constructs a geodesic for a sample non-constant choice for the background covector. The existence of these classical analogues demonstrates that Finsler $b$ spaces...

Topics: Classical Physics, Physics

Source: http://arxiv.org/abs/1504.07935

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Sep 23, 2013
09/13

by
Ralf Lehnert; Robertus Potting

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The emission of electromagnetic radiation by charges moving uniformly in a Lorentz-violating vacuum is studied. The analysis is performed within the classical Maxwell-Chern-Simons limit of the Standard-Model Extension (SME) and confirms the possibility of a Cherenkov-type effect. In this context, various properties of Cherenkov radiation including the rate, polarization, and propagation features, are discussed, and the back-reaction on the charge is investigated. An interpretation of this...

Source: http://arxiv.org/abs/hep-ph/0408285v1

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Sep 18, 2013
09/13

by
Alan Kostelecky; Ralf Lehnert

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Stability and causality are investigated for quantum field theories incorporating Lorentz and CPT violation. Explicit calculations in the quadratic sector of a general renormalizable lagrangian for a massive fermion reveal that no difficulty arises for low energies if the parameters controlling the breaking are small, but for high energies either energy positivity or microcausality is violated in some observer frame. However, this can be avoided if the lagrangian is the sub-Planck limit of a...

Source: http://arxiv.org/abs/hep-th/0012060v1

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Sep 20, 2013
09/13

by
A. J. Hariton; Ralf Lehnert

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The spacetime symmetries of classical electrodynamics supplemented with a Chern-Simons term that contains a constant nondynamical 4-vector are investigated. In addition to translation invariance and the expected three remaining Lorentz symmetries characterized by the little group of the external vector, the model possesses an additional spacetime symmetry if the nondynamical vector is lightlike. The conserved current associated with this invariance is determined, and the symmetry structure...

Source: http://arxiv.org/abs/hep-th/0612167v1

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Sep 19, 2013
09/13

by
Ralf Lehnert; Thomas J. Weiler

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We consider the neutrino (and antineutrino) flavors arriving at Earth for neutrinos produced in the annihilation of weakly interacting massive particles (WIMPs) in the Sun's core. Solar-matter effects on the flavor propagation of the resulting $\agt$ GeV neutrinos are studied analytically within a density-matrix formalism. Matter effects, including mass-state level-crossings, influence the flavor fluxes considerably. The exposition herein is somewhat pedagogical, in that it starts with...

Source: http://arxiv.org/abs/0708.1035v1

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Sep 20, 2013
09/13

by
Ralf Lehnert; Thomas J. Weiler

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The effect of the higher-energy 2nd resonance and the associated adiabatic-to-nonadiabatic transition on neutrino propagation in solar matter is presented. For WIMP-annihilation neutrinos injected with energies in the "sweet region" between 300 MeV and 10 GeV at the Sun's center, a significant and revealing dependence on the neutrino mass hierarchy and the mixing angle theta_13 down to 0.5 degrees is found in the flavor ratios arriving at Earth. In addition, the amplification of...

Source: http://arxiv.org/abs/1002.2441v1

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Jun 30, 2018
06/18

by
Mauro Cambiaso; Ralf Lehnert; Robertus Potting

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Asymptotic single-particle states in quantum field theories with small departures from Lorentz symmetry are investigated perturbatively with focus on potential phenomenological ramifications. To this end, one-loop radiative corrections for a sample Lorentz-violating Lagrangian contained in the Standard-Model Extension (SME) are studied at linear order in Lorentz breakdown. It is found that the spinor kinetic operator, and thus the free-particle physics, is modified by Lorentz-violating...

Topics: High Energy Physics - Theory, High Energy Physics - Phenomenology

Source: http://arxiv.org/abs/1401.7317

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Sep 18, 2013
09/13

by
Alan Kostelecky; Ralf Lehnert; Malcolm Perry

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Spacetime-varying coupling constants can be associated with violations of local Lorentz invariance and CPT symmetry. An analytical supergravity cosmology with time-varying fine-structure constant provides an explicit example. Estimates are made for some experimental constraints.

Source: http://arxiv.org/abs/astro-ph/0212003v1

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Jun 30, 2018
06/18

by
Mauro Cambiaso; Ralf Lehnert; Robertus Potting

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Radiative corrections in quantum field theories with small departures from Lorentz symmetry alter structural aspects of the theory, in particular the definition of asymptotic single-particle states. Specifically, the mass-shell condition, the standard renormalization procedure as well as the Lehmann-Symanzik-Zimmermann reduction formalism are affected.

Topic: High Energy Physics - Phenomenology

Source: http://arxiv.org/abs/1410.1345

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Sep 18, 2013
09/13

by
Mauro Cambiaso; Ralf Lehnert; Robertus Potting

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All quadratic translation- and gauge-invariant photon operators for Lorentz breakdown are included into the Stueckelberg Lagrangian for massive photons in a generalized \xi-gauge. The corresponding dispersion relation and tree-level propagator are determined exactly, and some leading-order results are derived. The question of how to include such Lorentz-violating effects into a perturbative quantum-field expansion is addressed. Applications of these results within Lorentz-breaking quantum field...

Source: http://arxiv.org/abs/1201.3045v1

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Sep 22, 2013
09/13

by
Orfeu Bertolami; Ralf Lehnert; Robertus Potting; Andre Ribeiro

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Many candidate fundamental theories contain scalar fields that can acquire spacetime-varying expectation values in a cosmological context. Such scalars typically obey Lorentz-violating effective dispersion relations. We illustrate this fact within a simple supergravity model that also exhibits the observed late-time cosmological acceleration and implies varying electromagnetic couplings.

Source: http://arxiv.org/abs/astro-ph/0310344v1

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Sep 23, 2013
09/13

by
Michael A. Hohensee; Ralf Lehnert; David F. Phillips; Ronald L. Walsworth

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The absence of vacuum Cherenkov radiation for 104.5 GeV electrons and positrons at LEP combined with the observed stability of 300 GeV photons at the Tevatron constrains deviations of the speed of light relative to the maximal attainable speed of electrons. Within the Standard-Model Extension (SME), the limit $-5.8\times10^{-12} \leq \tilde{\kappa}_{\rm tr}-{4/3}c_e^{00} \leq 1.2\times10^{-11}$ is extracted, which sharpens previous bounds by more than 3 orders of magnitude. The potential for...

Source: http://arxiv.org/abs/0904.2031v1

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Sep 18, 2013
09/13

by
Michael A. Hohensee; Ralf Lehnert; David F. Phillips; Ronald L. Walsworth

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We consider the possibility that Lorentz violation can generate differences between the limiting velocities of light and charged matter. Such effects would lead to efficient vacuum Cherenkov radiation or rapid photon decay. The absence of such effects for 104.5 GeV electrons at the Large Electron Positron collider and for 300 GeV photons at the Tevatron therefore constrains this type of Lorentz breakdown. Within the context of the standard-model extension, these ideas imply an experimental...

Source: http://arxiv.org/abs/0809.3442v2